I go out to start my car yesterday...it's cold out, but not COLD. Maybe 45 or 50 degrees. There have been much colder days over the past few weeks. So I start it up, and it runs fine for a second, and the idle drops by a hundred or two as normal, but then the car starts trying to stall out. It idles normal, then the idle drops drastically like maybe 400 rpms, then kicks back up again..it's doing this really rapidly..buck, die, buck, die.... So I keep my foot lightly on the gas to bring the rpms up to try to iron this out...but as soon as I let off, it does it again, so I let it go on its own and it stalls. I turn the key forward to let the fuel pump prime, as is my habit to do when it's cold out..I always let it prime for a second first...but the pump doesn't prime. I yank the key, insert it and turn it over again..the pump still doesn't prime. Never-the-less, the car starts..I assume it still had enough gas in its system, but...have you ever heard of a fuel pump just failing like that, and then working fine later? And did that odd incident and stall have anything to do with the fuel pump not priming afterwards, or is it just a strange coincidence? And no, it's NEVER done this before. Thanks guys.

 

Its better to die in the driveway then at 65mph going to work,about 10 miles from home!

When mine went...it was like some one just pulled the plug. Now I have problems with it NOT wanting to shut off. After a few shakes of the relay it quits. What you have could be a short in the relay, a loose ground wire are the unfortanant dieing fuel pump.

If you find a fuel pump change is in your future...below is the process which we changed mine.

When we replaced mine, we cut a 8x12 square hole in the floor above the tank to gain access. With the carpet pulled back..we started the cut about 3 inches below the top of the floor then 9 inches toward the rear. With the hole being 8 inches wide the entire length.
We then cut the metal fuel lines and used 4 inch rubber hoses to couple them back together with 4 clamps per hose to be sure of no leaks.
After in replacement we used a 9x13 piece of sheetmetel and some short machine screws to cover the hole. And a rubberized sealer to seal the area around the "lid".

About 7 months ago the spark module went out. The father in law swore up and down it was the fuel pump. The car would run until the mod would get hot. Since I have a life time warrenty we replaced the pump....just to make him happy(and to prove he was wrong lol).
Took about 10 minutes to change with the hole we made.

If you are thinking of this method just know that you have less then 1 inch of clearance between the body and the fuel tank.

But definatly get a 2nd opinion before doing anything drastic.

 

Sounds like the fuel pump. Id put a pressure tester on the fuel rail or disconnect the fuel filter and pop the tester on there and see how much pressure there is. If theres no pressure test the wires going back to the fuel pump, if you have voltage at the wires then the fuel pump is toast. If there's no voltage then start checking fuses.

Id bet the fuel pump is toast and its not a fuse or relay.

 

I agree with SSC... throw a fuel pressure gauge on it. (Got mine for $30 from Pep Boys, made by Sunpro, make sure to get the port fuel injection one and not the low-pressure $12 carburetor gauge!)

I believe if the relay died, all you would have is hard starting. When oil pressure built up, the oil pressure safety switch would've kicked on, and kept the pump running.

As to the pump not priming again; I know sometimes, I don't hear my pump prime if I turn the car off and then turn the key back to "on". (Glad to hear you let the pump prime up before cranking it!) So that might be "normal". The ideal thing to do would be wait 1/2 hour (probably much longer than necessary), then see if the pump still doesn't prime.

You have a 3.1, right? So you won't have a cold start injector like us 85-89 2.8 guys. I think the only other thing that could affect cold starting on your 90-92 speed density injection is the coolant temperature sensor. If you have a multimeter, throw it on, here's a chart of temperature vs resistance:

<table BORDER=3 CELLSPACING=3 CELLPADDING=3 >
<tr><td><b>Temp F</b></td><td><b>Temp C</b></td><td><b>Ohms</b></td></tr>
<tr><td>210</td><td>100</td><td>185</td></tr>
<tr><td>160</td><td>70</td><td>450</td></tr>
<tr><td>100</td><td>38</td><td>1,600</td></tr>
<tr><td>70</td><td>20</td><td>3,400</td></tr>
<tr><td>40</td><td>4</td><td>7,500</td></tr>
<tr><td>20</td><td>-7</td><td>13,500</td></tr>
<tr><td>0</td><td>-18</td><td>25,000</td></tr>
<tr><td>-40</td><td>-40</td><td>100,700</td></tr>
</table>

(The MAT, manifold air temp sensor, in the airbox, uses these same values, by the way. But I don't think the ECM reads the MAT to determine if it should enter "cold start" mode (extra fuel thru your injectors).

You might also have a leaking injector. I've had one since the spring... and nope, haven't replaced it yet. (Haven't had time to screw with the car in a lonnng while...) I thought the pump was dying, too... and almost got a heart attack. I put the pressure gauge on, and turned the key to on, waiting for the pump to prime up to 42 psi. It primed all right, and as soon as the pump shut off, pressure dropped right to zero. I thought "crap, the drainback valve is busted!" (That's in the pump.) Then I took out my trusty GM Service Manual (from http://www.helminc.com ) and ran thru the chart. Turned out that it pointed me exactly to a leaking injector- which one, I don't know. Here's my message with the full details, maybe it's the same symptom you're talking about? https://www.thirdgen.org/techbb2/sho...hreadid=136733

And here's another message about injectors: https://www.thirdgen.org/techbb2/sho...hreadid=139637

 

My car sounded like that when it needed the IAC valve changed. Sometimes I don't hear my fuel pump prime, but it alwasy starts up right away just the same. I would at least take the IAC valve out and clean it before I started working on a fuel pump. Just my humble opinion. Sorry to waste your time if you have already tried this.

 

Tom...I didn't know the 3.1's dropped the cold start injector? Hmm..guess it just wasn't needed. Also, the car has only done this once..never happened before, and it's never had any fuel deficiency problems except when my gas tank is low the car has a cycle of minor bogging...4 seconds of clean, 4 seconds of bog...at cruise only... And about the IAC..yes, I need to clean mine. Car stalled out at a stopsign right after that incident as a matter of fact...IAC for sure cause the fuel filter is brand new. I dont even know where the IAC valve is though..and I assume I can just use throttle body cleaner on it?

 

The PCM will compare the ECT and MAT values. If they are the same upon start-up, the PCM determines it's a "cold start".

 

For the laymen among us...MAT and IAT are the same thing just different names correct? And...the PCM..something Control Module, what's the P stand for? And..the ECT...I have no guess for that one...?

 

MAT=IAT
Manifold Absolute Temperature
Intake Absolute Temperature

PCM=ECM=VCM
Powertrain Control Module
Electronic Control Module
Vehicle Control Module

ECT=CTS
Engine Coolant Temperature
Coolant Temperature Sensor

 

Ohhhh ok gotcha...makes sense. I've heard IAT and MAT both used but for the others I only really hear ECM and CTS.

 

Thats about 7 degrees Celcius. I was wondering how cold it was over night. If it made it down to 32 F, then maybe you had some water that froze up in the gas lines? Just a thought around this time of year.

The other thought was maybe a loose ground wire.

If it does this againwhere the pump will not prim, tap the bottom of the gas tank, if the pump then kicks in after a few taps this means it's days are numbered and could go any time.

 

I doubt it made it down to 32...it wasn't even that cold that morning, and it's Florida...I'm lucky to see 40 degrees. Last night I went out at like 11:30 pm to 1 am and didn't even have a jacket..it was like 70 the whole time. Didn't even use the heat in my car. If it does this again, I'll try to check it out more....but like I said, this is the only time it's done this so far.

 

Yeah being in Florida you would probably never see freezing temperatures. Thats too bad, thats when all the real problems start . I thought the freezing idea would be something simple to throw in there. But if there are hardly ever freezing temperatures then its not a possibility. (Its -10*C today(whats that like 27*F maybe), so that is what I thought of that.)

 

Yeah.. Today I'm wearing shorts and an a-shirt, and all the windows on the house are open...what's that tell you about the weather?

 

Hey keep it down, I had to put on my carhart coveralls and a stick-em up ski mask just to pull my starter.

 

Guess what? Driving home at 9:30 at night..still in an A-shirt with pants..with the window down. I'd kill for some snow though......I miss it. (Born in Ohio)

 

Well, in 86, for GM, there's ONLY a MAT, CTS, and ECM. I think IAT started in '93 for the 4th gen f-bodies. I usually stick to the period-specific terminology for parts...

Isn't ECT a foreign car or a Ford thing?

 

most fuel pumps do that. if you would ahve waited a little bit to let the pressure get out of the lines it would have primed.

 

91, have a IAT :P

 

They changed it for '91?? No kidding! Thanks for the info; I was always under the impression that the IAT name came about in '93. So does that mean it's IAT in '90, as well?

Oh and are you sure the MAT gets checked by the computer if the car uses mass air technology? I thought MAF cars didn't check the MAT. GlennL98 mentioned something about that the MAT was only used for EGR purposes on the 2.8...